The present invention relates to improvements in or relating to data communication and particularly, but not exclusively, to improved protection for interleaved block codes such as are used in transmitting data messages, for example paging messages transmitted at various bit rates such as 2400, 3200 and 6400 bits per second.
Typically at the lower bit rates two level FM modulation, that is M=2, is contemplated but at the higher bit rates four level FM (Gray Coded) modulation, that is M=4, is contemplated. There is a relationship between the number, s, of bits in a symbol and the number, M, of modulation levels, namely M=2.sup.s. Taking four level FM Gray Coded modulation as an example, data is transmitted as 2 bit symbols and the symbols are transmitted as the symbol most significant bit (MSB) together with the symbol least significant bit (LSB). A problem which occurs in this mode of modulation is that the LSB is more susceptible to noise than the MSB because over the deviation range the MSB undergoes one transition whilst the LSB undergoes two transitions.
As is known, paging codes such as POCSAG and others are capable of limited error correction and in order to counter the effects of burst errors, block interleaving is utilised. As an example, 16 successive code words are arranged row by row to form a block and are transmitted column by column, beginning with the first bits of the first, second and third code words and so on until the first bit of the sixteenth code word, then continuing with the second bit of the first code word and so on until the last bit of the 16th code word. Using Gray Coded modulation schemes, the even numbered bits as transmitted are more susceptible to noise than the odd numbered bits. The practical effect of this is that at the receiver when the blocks of 16 code words are reconfigured, the odd numbered code words are more dependable than the even numbered code words.
The ERMES paging code published by ETSI, the European Telecommunications Standards Institute, contemplates a single bit rate. Block interleaving is used but each block comprises an odd number of code words, namely 9 code words. A practical effect of this is that, when transmitting data symbols are read out as before, but because of there being an odd number of code words in the block, when reading out the even numbered columns the odd numbered bits are inherently more susceptible to noise than the even numbered bits. Consequently, when the blocks of code words are re-established at a receiver each code word comprises bits which are more dependable interleaved with bits which are less dependable.
Higher rate paging protocols now being considered require a basic framing structure in which batches or blocks are of constant duration which means that if the bit rate is doubled then double the number of code words comprise the interleaved block structure. The improved bit protection provided by ERMES would under such circumstances not be possible because each interleaved block would comprise an even number of code words if the bit rate was doubled or quadrupled and as a result the transmission of symbols would result in alternate reconstituted code words being of greater and less dependability.